1. Field of the Invention
Priority is hereby claimed to Japan Patent Application No. 2012-218762 filed on Sep. 28, 2012, and such priority application is hereby incorporated by reference herein, in its entirety. The present invention relates to an optical modulator, in particular, relates to the optical modulator comprising a substrate having a thickness of 20 μm or less and an electric-optic effect, a reinforcing substrate holding the substrate thereon, and a resin layer disposed between the substrate and the reinforcing substrate.
2. Description of Related Art
An electro-optic crystal such as lithium niobate (LN) is used and a travelling-wave type modulator in which an optical waveguide with a Mach-Zehnder (MZ) structure is formed on the crystal substrate is widely used for optical modulation in a technical field of an optical communication or an optical measurement.
In a case where a substrate constituting an optical modulator is thinned to approximately 20 μm, it is possible to achieve velocity matching of light waves propagating through an optical waveguide and modulation signals propagating through a control electrode without forming a buffer layer formed of SiO2 and the like between a substrate on which an optical waveguide is formed and a control electrode. Thus, it is possible to obtain an optical modulator in which drive voltages are reduced.
On the other hand, an optical modulator in which a plurality of MZ structures are integrated is also used in order to deal with various modulation formats. For example, as disclosed in International Publication No. WO2011/004615 or Japanese Laid-open Patent Publication No. 2011-034057, an optical modulator having an insert die-type optical waveguide in which a sub-Mach-Zehnder type optical waveguide is incorporated into two branch waveguides of a main Mach-Zehnder type optical waveguide, so-called, a nest-type optical waveguide is also used. The nest-type optical modulator particularly receives attention as an optical modulator which is capable of performing a high-speed operation such as a DP-QPSK modulator.
A gap between the two branch waveguides in the main Mach-Zehnder structure is as wide as approximately 100 μm to 500 μm in such a nest-type optical modulator into which the MZ structures are integrated.
As illustrated in FIGS. 1A and 1B, a sub-Mach-Zehnder type optical waveguide 2 is formed in each branch waveguide 1 of the main Mach-Zehnder type optical waveguide.
Reference numerals 3 and 30 of FIG. 1A are control electrodes which control a phase of an optical wave propagating through a branch waveguide 1 by applying a predetermined electric field to the branch waveguide 1. DC bias voltages are applied to the control electrodes 3 and 30 in order to hold the optical waves propagating through each branch waveguide in a predetermined phase difference. FIG. 1B is a cross-sectional view taken along an alternate long and short dash line A-A of FIG. 1A. In FIGS. 1A and 1B, an X-cut-type substrate is used as a substrate 4; however, even in a case where a Z-cut substrate is used, there is a case where a control electrode 30 straddling over the two branch waveguides 1 is formed.
The control electrode is formed to straddle over the branch waveguides and has a width of several hundreds of μm in the same manner as the gap between the branch waveguides. However, as illustrated in FIG. 1B, if the width of the control electrode disposed between the branch waveguides 1 becomes larger, since the width of the control electrode 30 is larger in comparison with the thickness of the substrate 4, an electric field distribution 7 generated between the control electrodes is widened. As a result, the electric field is distributed to the outside of the substrate 4.
As illustrated in FIG. 1B, in a case where the substrate 4 becomes thinner, a reinforcing substrate 6 is bonded via a resin layer 5 such as an adhesive in order to reinforce the substrate 4. The electric field distribution 7 extends to a range of the resin layer 5. In a case where high-frequency modulation signals are applied to the control electrode, since the electric field distribution formed by the modulation signals is limited to a vicinity of the electrode, the electric field distribution does not penetrate into the resin layer 5, unlike as illustrated in FIG. 1B. However, in a case where DC voltages such as DC bias voltages are applied to the control electrode, as illustrated in FIG. 1B, the electric field is distributed to the outside of the thin substrate 4.
In a case where an electric field is distributed to the resin layer, a modulation curve of the optical modulator is easily shifted, so-called, a drift phenomenon becomes significant as compared with a case where an electric field is not widely distributed to the inside of the resin by characteristic changes due to mobile ions in the resin or alternation and degradation of the resin layer. If the DC bias voltages to be applied are increased in order to control bias points of the modulation curve, the electric field further penetrates into the resin layer 5. Therefore, characteristic degradation of the optical modulator is more accelerated.
On the other hand, it is possible to configure the control electrode 30 after a shape of the branch waveguide 1 is changed and the gap between the branch waveguides is narrowed. However, in this case, a conversion unit for changing the gap between the waveguides is necessary, and as a result, element length is increased.